An operation mode of a car or a train is characterized such that the car or train runs (coasts) by inertia without accelerating and decelerating by a driving system. In a driving system that uses synchronous machines, in a case of such coasting, a no-load induction voltage is generated, this no-load induction voltage is subjected to full-wave rectification via diodes anti-parallel connected to switching elements that constitute a power converter, and a direct-current voltage (a voltage between terminals of a capacitor) rises. Therefore, electric power that has been full-wave rectified is regenerated to a power supply side, and the driving system performs a brake operation as a whole.
As means for avoiding such a brake operation, a power conversion device disclosed in Patent Literature 1 mentioned below is configured as follows. That is, in the power conversion device, a parallel connection circuit that includes a one-way conduction unit and a switching unit is inserted in series between the power supply of an inverter and an inverter arm. The switching unit is released while the inverter stops, and an excitation current is controlled so that the terminal voltage of a synchronous machine is equal to a predetermined value by releasing the switching unit when the inverter starts operating. When the terminal voltage of the synchronous machine reaches the predetermined value, the power conversion device controls a torque current of the synchronous machine in a state of closing the switching unit to cause the synchronous machine to perform an acceleration and deceleration operation. Furthermore, to control the inverter to stop operating, the power conversion device reduces the torque current to zero while controlling the excitation current so that the terminal voltage of the synchronous machine is equal to the predetermined value, and then releases the switching unit and reduces the excitation current to control the inverter to stop operating.
Furthermore, Patent Literature 2 mentioned below discloses a main circuit configuration related to a power conversion device that drives a plurality of synchronous machines.